This invention relates to apparatus for forming plastic articles, and, more particularly, relates to such apparatus for forming plastic articles by a thermoform process.
In the forming of plastic objects by a thermoform process, it is conventional to provide a roll of plastic stock material to be formed into the desired object. A sheet of the material is pulled outwards from the roll and disposed over a die or mold of the object to be formed and then sealed along the periphery of the mold. The plastic sheet is then heated to a point where it becomes pliable. Next, either positive pressure is applied from above the sheet forcing it into the conforms of the mold or, in the alternative, a vacuum is drawn from below the sheet evacuating the air in the space between the mold surface and the surface of the sheet thereby, in like manner, drawing the surface of the sheet into the conforms of the mold. The mold is typically water cooled, whereby upon contact of the heated plastic against the cool mold, the plastic sets and may thereafter be removed from the mold. The molded plastic is thereafter trimmed by a die cutting process or the like whereby the scrap plastic from the sheet is trimmed away from the desired object.
One problem with such techniques is that the object to be formed is not precut to shape from the plastic roll. Accordingly, virtually the entire forming process relies on the vacuum or positive pressure to draw the plastic about the mold, and thus the shape of the plastic sheet prior to heating does not contribute to the desired object's final shape. Moreover prior techniques, in relying upon the vacuum or pressure, often required sealing or some means of restraining the edges of the plastic sheet to be formed. Thus, beneficial effects of gravity on the edges of the heated sheet in assisting formation of the desired object were neglected.
Accordingly, the present invention overcomes the hereinbefore described problems as well as others, providing a novel method and apparatus for thermoforming plastic articles.